1. Technical Field
This invention relates to a control device for an internal combustion engine and a continuously variable transmission. The control device is connected to the continuously variable transmission, which is disposed between the internal combustion engine mounted on a vehicle and driving wheels, and changes and controls a transmission ratio of the continuously variable transmission at a transmission ratio changing speed suitable for driving conditions of the vehicle while controlling an output of the internal combustion engine.
2. Description of Related Art
Generally, engine output (hereinafter, referred merely as output) of an internal combustion engine (hereinafter, referred merely as an engine) mounted on a vehicle is mechanically controlled by a throttle device coupled to a driver-operable member such as an accelerator pedal and a throttle lever (which are representatively referred to as accelerator pedal hereinafter) through an accelerator cable.
The accelerator pedal and a throttle device cooperate with each other such that the displacement corresponding to a stepping amount of the accelerator pedal is transmitted to the throttle device, and a throttle valve in the device is actuated according to the displacement (stepping amount). Unfortunately, excessive output may be generated due to careless driving and lack of skill of a driver. Consequently, a vehicle may slide on starting, slip on icy ground, and skid (slip) upon sudden acceleration.
Accordingly, methods have been proposed such as a dual throttle valve method where a main throttle valve and a sub throttle valve are arranged in the throttle device. The sub throttle valve is electronically controlled via a traction control (power control) method utilizing the so called drive-by-wire method. In this method, the accelerator cable is not disposed between the accelerator pedal and the throttle valve, and an opening of the accelerator pedal is detected using a sensor such as a potentiometer. The throttle valve is then operated by a stepping motor or the like based on output of the sensor.
In the traction control method of the type described, an ECU (engine control unit) generally calculates an optimum opening (i.e., target engine output) for the sub throttle valve and the main throttle valve in accordance with the (1) data representative of the rotation condition of the front and rear wheels and (2) a step amount for the accelerator pedal. The ECU controls a driving torque of the wheels in a range which does not cause undesirable skid.
Information regarding the required output of the engine is properly set in accordance with, for example, the opening of the accelerator pedal. As mentioned above, the ECU calculates and sets the required output of the engine when using the traction control method for controlling the sub throttle valve and the main throttle valve so as to obtain the required output. In this event, it is preferable to carry out the calculation of the required output with respect to an actual torque under current engine conditions. More specifically, by calculating a deviation between a required torque and the actual torque and by carrying out a real-time control to eliminate the deviation, it is possible to prevent over-control and poor response of the control device.
While, the actual torque of the engine can be detected under a bench test using a chassis dynamometer, it is difficult in practice to mount the dynamometer on a vehicle due to the weight, size, and costs of the device. Plus, there is a serious defect that output (energy) loss is inevitably caused.
Accordingly, it is assumed that precision of the output control can be improved by calculating the actual torque in accordance with intake air flow information by using a conventional control system.
One power transmission method for transmitting output torque of the engine to wheels is a variable transmission. As one such transmission, a continuously variable transmission (CVT) can continuously change the transmission ratio by using a steel belt and pulleys, and can increase or decrease the transmission ratio changing speed depending on a hydraulic value supplied to a hydraulic actuator.
In the continuously variable transmission of the type described, the transmission ratio changing speed is calculated so as to eliminate a transmission ratio deviation between an objective transmission ratio, which is calculated in accordance with the driving conditions, and an actual transmission ratio. The hydraulic actuator of the continuously variable transmission is controlled in order to obtain the transmission ratio changing speed.
Problems to be solved by the present invention are as follows.
The conventional continuously variable transmission CVT mounted on a vehicle controls the hydraulic actuator so as to correct the actual transmission ratio to the object transmission ratio. However, a level of the torque to be transmitted is not taken into consideration.
Accordingly, when the transmission ratio of the CVT increases or decreases at a relatively low speed, the torque on the driving shaft is changed smoothly. On the other hand, when the transmission ratio deviation between the target transmission ratio and the actual transmission speed is relatively large, a transmission ratio changing has to be raised extensively. However, the continuously variable transmission CVT may cause a reduction in vehicle acceleration at extremely fast transmission ratio changing speeds on kick-down. Such a phenomenon is remarkable when the engine torque is small.
The continuous variable transmission CVT has a relatively large moment of inertia due to the pulleys. This negatively affects the transmission ratio changing speed with undesired excessive shock on transmission. In addition, excessive torque of the engine may result in sliding of a steel belt. Thus, the transmission ratio changing speed, which would lead to poor performance of the continuously variable transmission CVT, is restricted.